Two-port or two-way valves of the aforedescribed type, i.e. having a valve housing receiving the spring-biased differential piston and formed with the main fluid inlet, the main fluid outlet and a connection to a hydraulic control unit have been provided heretofore as built-in cartridge valves in hydraulic fluid technology, i.e. for hydraulic systems of various types. Such systems generally include a main pump or source of the hydraulic fluid, a hydraulic load such as a hydraulic motor or jack, and a hydraulic drain or reservoir into which the circuit empties and from which the hydraulic fluid is drawn by the pump.
The two-port valves with which the invention is concerned may be built into any of the control units of such a circuit or may be provided as a replaceable cartridge in such a unit and generally has a hydraulic network or valve set connected with the control port and at which a pressure is developed or established to shift the valve member or slider and thereby control the main fluid flow from the main inlet port to the main outlet port.
Such valves are utilized in hydraulic circuit technology for the digital (on/off) or analog (continuous-parameter) regulation of liquid streams.
In conventional two-port valves of this type, the differential-pressure-control element generally is a solid cylinder whose action can be stabilized only by fixed damping throttles for analog control. It is practically impossible for a single such element to provide satisfactory pressure control or flow rate control at least in part because in conventional valves of this type the main flow throttling affects the flow of liquid in the control circuit and hinders or precludes valve closure or switchover from an open to a closed state.
In German patent No. 1,927,883 a hydrostatic amplifying system is provided which utilizes a differential piston and which does not have this latter disadvantage although it does have another disadvantage in common with all of the above-described earlier systems, namely, the lack of a well-defined relationship between the position of the throttle slider or member and the controlled flow or pressure.
Another disadvantage of earlier systems is that close tolerance manufacture of the components is necessary.